Borehole seismic survey systems may involve sources located at the surface and receivers placed in the well. Other configurations are possible, for example the drill bit can function as the seismic source and receivers can be placed at the surface. No matter the configuration, noise can be recorded in acquisition, which may impact the accuracy of seismic analysis.
Borehole seismic acquisition systems can be configured to avoid noise, for example by spacing sensors according to signal sampling requirements and by isolating sensors using mechanical design principles. For example, in borehole seismic acquisition, the downhole tool may include an array (or multi-level in depth) of individual acquisition nodes or shuttles housing at least one seismic sensor and associated acquisition electronics. At each depth level, these shuttles can be designed to optimize their geophysical coupling response to the surrounding formation by mechanical, magnetic or hydraulic clamping devices. Coupling to the formation may ensure the sensor(s) in the shuttle respond to the particle motion of the seismic wave passing through the formations penetrated by the borehole. Coupling can also serve to minimize the recording of geophysical noise, e.g. the tube wave energy in a borehole. Because such devices may be intended to record the seismic response in the surrounding formation (the geophysical signal) the depth interval between measuring points or shuttles can be defined by the seismic signal processing requirements. This depth interval may be on the order of ten meters. For example, the Schlumberger Versatile Seismic Imager (VSI)1 tool uses a 15 m separation with up to 40 separate shuttles, and the Paulsson Inc. 100-level array tool (Paulsson) also has a standard shuttle separation of 15 m. Both these tools are based on individual clamped units containing three-component geophones/accelerometers. 1 Versatile Seismic Imager (VSI) is a trademark of Schlumberger.
Clamping devices use power and are relatively heavy and so this may limit the number of measuring points and therefore the total length of the tool. In order to cover a larger depth aperture in the borehole, or to sample the depth interval more finely, the complete borehole tool should be moved to a different depth and the seismic experiment repeated, a procedure which relies on the repeatability of the seismic source signature. Clamping devices can also become stuck or jammed, increasing the risk of not being able to retrieve the tool from the borehole.
Alternative systems have been proposed and used in the past. U.S. Pat. Pub. No. 2008/0316860, which is herein incorporated by reference in its entirety, describes a borehole acquisition system (a single length of ‘streamer’) that contains hydrophones only. However, the system uses densely sampled groups of hydrophones to estimate gradients of the wavefield directly from the hydrophone measurements. The distance (depth interval) between these groups of hydrophones is governed by the signal sampling requirements.
Borehole streamers containing only hydrophones have been used to acquire both vertical seismic profile (VSP) and cross-well seismic surveys (see for example, Wong et al., TLE, January 1987, 36-41; Marzetta et al., A Hydrophone Vertical Seismic Profiling Experiment, Geophysics, 1988, 53 (11), 1437-1444; Kragh et al., Anisotropic Traveltime Tomography in a Hard-Rock Environment, First Break, 1995, 14 (10), 391-397). Although these systems were designed for borehole environments they appear to have limited overall length (32 channels, 12 channels and 16 channels, respectively). The hydrophone spacing in these tools is a few meters, with the exception of Marzetta et al. (1988), which uses a 1.5 m hydrophone spacing to sample the tube wave noise adequately for later attenuation in processing, and 8 separate downhole positions to cover the aperture. Marzetta et al. (1988) notes that the modest degree of tube-wave attenuation achieved in their experiment was due, in part, to poor tube-wave coherency caused by shot-to-shot non-repeatability. These systems appear to not contain vector wavefield sensors.